Melt Butter – In a medium saucepan over medium heat, melt the butter.
Make Roux – Add flour and whisk continuously for 1-2 minutes until bubbly and golden.
Add Milk – Gradually pour in milk while whisking to avoid lumps. Cook for 3-4 minutes until slightly thickened.
Add Cheese – Reduce heat to low and stir in shredded cheese until fully melted and smooth.
Season – Add salt, pepper, paprika, and mustard powder (if using). Adjust seasoning to taste.
Serve – Use immediately over nachos, pasta, or vegetables.